1. Technical Field
The present invention relates to a technique for detecting whether a display screen is touched with a detection target medium or not by means of an optical detection method.
2. Related Art
As an example of the recent advancement of optical detection technology, a display device having an optical-detection input function has been developed in the technical field to which the present invention pertains. A display device of the related art that has an optical-based input capability is provided with optical sensors, each of which is provided for one pixel or a group of pixels. With these optical detectors, the related-art display device such as a liquid crystal panel, though not limited thereto, enables various kinds of information to be inputted through the display screen thereof. The optical sensor provided in the pixel of the related-art display device has, as an example of constituent elements thereof, a photodiode and a capacitor. In such a configuration, the quantity of electric charge of the capacitor changes in accordance with the amount of light received at the photodiode. On the basis of the detected voltage of each terminal of the capacitor, the amount of light received at the photodiode can be detected. In an effort to improve the accuracy in judgment made as to whether the display screen is (i.e., was) touched with a finger or not and to improve the accuracy in the calculated coordinate position thereof, an image-edge optical detection technique has been proposed, an example of which is disclosed in JP-A-2006-244446. In the related art described in JP-A-2006-244446, the edge of a picked-up (i.e., “photographed”) image is detected. Then, on the basis of the acquired edge image, a judgment is made as to whether an object contacts the display surface or not.
In the above-identified related art of JP-A-2006-244446, a touch operation is detected on the basis of the “swelling” (i.e., enlargement) of a finger due to pressure applied at the time when a user touches a panel surface with the finger. For this reason, the above-identified related art of JP-A-2006-244446 cannot detect the touching of a display surface with any hard inelastic object (i.e., detection target medium) that cannot be deformed, unlike the finger, at the time of the touching thereon.